In equipment such as a thermal power station where fossil fuel or waste is used as an energy source by burning, when fuel such as tar, coal or heavy oil of poor quality is used, or when plastics or waste including a great deal of salt is used as fuel, the combustion products of such fuel, etc. often contain a great deal of Na, K, S and Cl. Low melting point compounds containing sulfates and chlorides such as Na.sub.2 SO.sub.4, NaCl, KCl and CaSO.sub.4 are formed on the surface of furnace wall tubes, steam superheater tubes, etc. of power generation equipment or combustion equipment. Consequently, scale formed on the tube surface melts to produce hot corrosion. Long term use of the furnace wall tubes and steam superheater tubes thus results in their destruction.
Furthermore, in a coal burning boiler, or waste incineration equipment or thermal power station of fluidized bed furnace type, hot erosion caused by combustion ash and fluid sand is produced on the surface of steam superheater tubes, etc., and accelerates hot corrosion.
The literature on related techniques (e.g., Corrosion, 42, 568 (1986); Iron and Steel, 67, 996 (1981)) illustrates that alloy components such as Cr have a corrosion resistance to a certain degree to such types of corrosion, particularly to coal ash corrosion characterized by sulfate corrosion. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 58-177438 discloses an austenitic stainless steel having an improved high temperature corrosion resistance.
For high temperature corrosion in a chloride environment, Japanese Examined Patent Publication (Kokoku) No. 64-8695 discloses that alloying components such as Mo, W and V are effective for cooking electrical appliances in an environment where foods containing chlorides are heated. The Patent Publication also suggests that the alloy is also suited to a steel tube for an incineration boiler used in a NaCl-containing environment. However, a boiler tube is required to have a certain strength as a pressure-resistant member, and notwithstanding the constituent system of the alloy in which an embrittlement phase such as the .sigma.-phase is extremely liable to precipitate, no concrete countermeasures have ever been shown. In addition to this, the Si content in the examples is up to 1.3% although the upper limit of the content of Si which is thought to be extremely effective for corrosion resistance but which is thought to accelerate precipitation of the .sigma.-phase is stipulated to be 2.0%.
The present inventors have heretofore proposed in Japanese Unexamined Patent Publication (Kokai) No. 7-243007 an alloy and a multilayer steel tube having corrosion resistance to a certain degree in an environment where hot corrosion is produced. However, since the alloy and the steel tube disclosed therein have a relatively high Ni content, they tend to show an increased corrosion amount in an environment where the concentration of sulfates is high. Moreover, the component conditions such as a Ni content are not necessarily preferable, and, therefore, a further improvement of the alloy and the steel tube is required.